Method for electrodepositing polonium



METHOD FOR ELECTRODEPOSITING POLONIUM Ralph F. Wehrm'ann, Waukegan, 111., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Mar. 16, 1950, Ser. No. 150,101

2 Claims. (Cl. 204-45) This invention relates to metallic polonium and more particularly to depositing a thick, uniform layer of polonium metal from aqueous solutions of polonium.

Polonium is a naturally radioactive element which emits an alpha particle and has a half life of approximately 140 days. Suitably prepared it can serve because of its very short half life, as an intense source of alpha particles. For such use it is important that the polonium be in the form of a film of uniform thickness since unevenness in a film may cause adsorption of as much as 80 to 90 percent of the emitted particles.

It is an object of the invention to obtain a smooth foil of even thickness of polonium suitable for use as an alpha particle source.

A further object of the invention is to provide means for obtaining a relatively thick foil of polonium.

A further object is to provide a method by which polonium can be rapidly recovered from an aqueous solution.

A still further object is to provide a means for obtaining extremely smooth deposits of polonium upon a base material. Other objects will appear hereinafter.

These objects are accomplished by electroplating polo nium from an aqueous solution containing hydrofluoric acid and polonium salts in solution. The invention may be best explained by reference to specific experiments performed.

Example I A platinum foil coated with about 1.3 curies of polo nium, deposited from 1.5 N HNO at 0.0 volt, was placed in mil. of 1 N HF. In order to keep oxidation to a minimum, the foil was removed from the nitrate plating bath, quickly rinsed in water and immediately immersed in the HF solution. On standing overnight practically all of the polonium dissolved (less than 1 millicurie was found on the platinum). The resulting solution, contained in a Lucite beaker, was colorless, and no evidence of a precipitate was observed.

The solution was electrolyzed at room temperature and at a cathode potential of 0.0 volt (vs. a normal calomel electrode). The electrodes consisted of a platinum anode of approximately 1 cm? total area and a platinum cathode. The plating area of the latter was .d to 0.25 cm? (by coating a portion of the surface with an electrical resist agent). A cathode potential controller was used to maintain the cathode potential. The rate of deposition was followed with an alpha counter. The electrodes were cleaned in an alkaline cleaner and rinsed in dilute HNO and water just prior to the start of electrolysis. As the electrolysis proceeded practically no change in the appearance of the cathode was observed, although an alpha count indicated zssmza Egg Patented Au so, 1960 the solution was being depleted of activity at a reasonable rate. The electrolyzing voltage and also the current remained practically constant throughout the plating period. The magnitude of the former was approximately 0.60 volt and that of the latter about 0.3 milliampere (1.2 amps./ft.

At the end of 8.5 hours of plating the cathode was removed as the alpha count indicated that the solution had been depleted of activity. The cathode remained very bright, so much so that it was 'diliicult to detect the presence of a deposit. After rinsing in water the cathode was immersed in dry alcohol and transferred to the dry box where it was dried in a nitrogen atmosphere and placed in a viewbox. The deposit darkened considerably on standing overnight, probably due to traces of moisture and/or oxygen in the nitrogen atmosphere. Under the microscope the deposit appeared ex tremely smooth even at 60x magnification.

The density of the deposit was determined as about 4.76 curies/cm.

Example II An aqueous solution of 1.0 N HF, 0.001 molar in respect to its polonium concentration, was prepared. This solution was subjected to electrolysis utilizing platinum electrodes. The cathode potential was maintained at 0.0 volt (vs. a norm-a1 calomel electrode) and the current density was maintained at about 2 milliamps./ cm. After continuing the electrolysis for 4 /2 hours, a plate thickness of polonium of 39.7 curies/cm. was obtained. The deposit had a very smooth appearance.

The operable ranges of the invention are quite broad. Aqueous hydrofluoric acid solutions ranging-in HF concentrations from 0.2 normal through 48% by weight (approximately 27 N) may be used, and the initial polonium concentration may range from less than one millicurie/ml. to saturated solutions. Current densities up to more than 10 milliamps./cm. give satisfactory deposits.

By the method of the present invention deposits of polonium of a thickness of more than 40 curies/cm. have been obtained. This is a great improvement over what was possible by the methods previously used. For example, in the electrodeposition of polonium from aqueous HNO solution, the concentration of polonium deposited is limited to 7.6 millicuries per square centimeter. (Seaborg, Katz and Manning, The Transuranium Elments, Part II, pages 1174-1175, National Nuclear Energy Series, McGraw-Hill, New York, 194-9).

The process has an additional advantage in that the solubility of polonium is higher in hydrofluoric acid solution than in other acid solutions so that more polonium can be plated out of a single run. A further advantage is that polonium may be plated at a faster rate from aqueous hydrofluoric acid solution than from other solvents.

As observed in Example I the method of the invention has in addition to the advantages recited above the further advantage of obtaining a very smooth deposit. The foils obtained by this method are efficient alpha sources and are useful as sources of alpha particles for calibration and similar purposes.

What is claimed is:

l. The method of obtaining a thick, smooth foil of polonium which comprises electrolyzing an aqueous solution of l N hydrofluoric acid containing about 0.13 curie of polonium per cubic centimeter of solution, said electrolysis employing platinum electrodes and a current density of about 1.2 milliamperes per square centimeter a 4 References Cited in the file of this patent UNITED STATES PATENTS 2,254,170 Dillon Aug. 26, 1941 OTHER REFERENCES Coche et a1.: Chem. Abstracts; vol. 40, September- November 1946, 001. 6346 

1. THE METHOD OF OBTAINING A THICK, SMOOTH FOIL OF POLONIUM WHICH COMPRISES ELECTROLYZING AN AQUEOUS SOLUTION OF 1 N HYDROFLUORIC ACID CONTAINING ABOUT 0.13 CURIE OF POLONIUM PER CUBIC CENTIMETER OF SOLUTION, SAID ELECTROLYSIS EMPLOYING PLATINUM ELECTRODES AND A CURRENT DENSITY OF ABOUT 1.2 MILLIAMPERES PER SQUARE CENTIMETER OF CATHODE SURFACE. 